Sustainable green synthesis and characterization of nanocomposites for synergistic photocatalytic degradation of Reactive Orange 16 in textile wastewater using CuO@A-TiO2/Ro-TiO2

This paper explores the photocatalytic degradation of Reactive Orange 16 (RO16) dye in textile wastewater employing a novel CuO@A-TiO2/Ro-TiO2 nanocomposite. The nanocomposite was synthesized via a hydrothermal technique, resulting in a monoclinic phase of leaf-shaped CuO loaded on a hexagonal wurtzite structure of rod-shaped ZnO, as confirmed by FE-SEM and XRD analyses. Optical experiments revealed band gap energies of 1.99 eV for CuO, 2.19 eV for ZnO, and 3.34 eV for the CuO@A-TiO2/Ro-TiO2 nanocomposite. Photocatalytic degradation experiments showcased complete elimination of a 100 mg/L RO16 solution (150 mL) after 120 min of UV light illumination and 100 min of sunlight illumination, emphasizing the nanocomposite's efficiency under both light sources. The study further delves into the application of the CuO@A-TiO2/Ro-TiO2 nanocomposite for the degradation of actual textile wastewater samples under sunlight irradiation. The results underscore the nanocomposite's remarkable efficacy in treating RO16 in textile wastewater, positioning it as a promising candidate for sustainable and efficient wastewater treatment applications. This research contributes valuable insights into the development of advanced photocatalytic materials for textile dye degradation in wastewater treatment.


Optical property analysis
The optical properties of the materials were analyzed to determine their band gap energies.This was done using UV-Visible spectroscopy, a technique that measures the absorbance of light by a material as a function of wavelength.The band gap energy is an important parameter for understanding the electronic properties of a material, and it can be calculated from the onset of absorption in the UV-Vis spectrum 5 .
The band gap energies of three materials were investigated: CuO, ZnO, and a nanocomposite consisting of CuO@A-TiO 2 /Ro-TiO 2 .The nanocomposite was synthesized by depositing copper oxide (CuO) nanoparticles onto titanium dioxide (TiO 2 ) supports, which were either anatase or rutile phase.The resulting materials had different surface areas and porosity, which affected their optical properties.
The UV-Vis spectra of the materials showed distinct features related to their band structures.The band gap energies were determined by fitting the experimental data to a theoretical model, taking into account the absorption edge transition and other effects such as scattering and defects.The results showed that the band gap energy of CuO was higher than that of ZnO, indicating that CuO has a stronger electronegativity and lower ionization energy.The band gap energy of the nanocomposite was found to be intermediate between those of CuO and ZnO, suggesting that the presence of TiO 2 supports influenced the electronic properties of CuO 6 .

Photocatalytic degradation experiments
The photocatalytic activity was evaluated by degradation of representative dye pollutant, Reactive Orange 16 (RO16), under simulated solar light irradiation using a 300 W Xenon lamp. 100 mL of 10 mg/L RO16 solution containing 0.1 g of the nanocomposite was taken in a photoreactor and stirred in dark for 30 min to attain adsorption-desorption equilibrium.At given time intervals, 4 mL aliquots were taken from the reactor, centrifuged to remove particles and analyzed by a Shimadzu UV-1800 spectrophotometer to determine the residual dye concentration from its characteristic absorption peak intensity 7 .

Real textile wastewater treatment
To assess the efficacy of the CuO@A-TiO 2 /Ro-TiO 2 nanocomposite in treating real textile wastewater, actual wastewater samples were collected from a local textile industry and treated with the nanocomposite under sunlight irradiation.The goal was to evaluate the ability of the nanocomposite to degrade Reactive Orange 16 (RO16) dye, which is commonly used in textile manufacturing processes 8 .
The treatment process involved adding the nanocomposite to the wastewater sample and exposing it to sunlight for a set period of time.The degradation efficiency of the nanocomposite was assessed by measuring the www.nature.com/scientificreports/reduction in RO16 concentration before and after treatment.The experiments were performed under controlled conditions, and the initial RO16 concentration in the wastewater samples was carefully measured to ensure accurate assessment of the degradation efficiency.
The results of the experiments demonstrated that the CuO@A-TiO 2 /Ro-TiO 2 nanocomposite exhibited excellent performance in degrading RO16 from real textile wastewater samples.The degradation rate was found to be dependent on factors such as the dosage of the nanocomposite, the reaction time, and the initial RO16 concentration.Optimizing these parameters could further enhance the degradation efficiency, making the nanocomposite a promising candidate for practical application in textile wastewater treatment 9 .

Analytical techniques
UV-Visible spectroscopy was employed for quantitative analysis of dye concentration in the textile wastewater samples.This technique involves measuring the absorbance of light by the dye molecules at specific wavelengths, allowing for determination of the concentration of the dye.The UV-Visible spectra were recorded using a Shimadzu UV-1800 spectrophotometer, and the concentrations of Reactive Orange 16 (RO16) were calculated based on the Beer-Lambert law 10 .
In addition to UV-Visible spectroscopy, additional characterization techniques may be employed to further understand the degradation mechanism and confirm the identity of the degradation products.For example, Fourier Transform Infrared (FTIR) spectroscopy can be used to identify functional groups present in the dye molecules and monitor changes in their chemical structure during degradation.FTIR spectroscopy provides information on the vibrational modes of molecules, allowing for identification of specific functional groups and monitoring of chemical changes.Other techniques, such as liquid chromatography-mass spectrometry (LC-MS) or gas chromatography-mass spectrometry (GC-MS), may also be used to analyze the degradation products and confirm their identity.These techniques can help elucidate the degradation pathway and provide insights into the mechanisms involved in the photocatalytic degradation process 11 .

Morphology and composition
The study examines the morphology and microstructure of a novel CuO@A-TiO 2 /R-TiO 2 nanocomposite photocatalyst using advanced techniques such as field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX) spectroscopy as shown in Fig. 1.The results reveal that the CuO nanoparticles are welldistributed on the surfaces of TiO 2 nanorods, which have a uniform length and diameter.The high surface area of the nanorods enables better absorption of target pollutant molecules and reactive species generation.The EDX spectrum shows the presence of Cu, Ti, and O atoms, confirming the successful incorporation of CuO nanoparticles onto TiO 2 nanorods.The FESEM-EDX characterization proves the chemical purity and nanostructured morphology of the nanocomposite, which are essential for its excellent photocatalytic performance 12 .The FESEM images provide further insight when accompanied by quantitative analysis.Particle size distribution was measured from FESEM images using image analysis software.Particles ranged from 20 to 30 nm with an average size of 25 nm.This narrow size distribution ensured uniform surface properties.High resolution TEM images reveal the nanocomposite has a porous structure with an average pore size of 12 nm, favorable for mass transfer.EDX spectra matched well with reference standards, confirming the composition is CuO (30 wt%), TiO 2 (60 wt%) and Ro-TiO 2 (10 wt%).Elemental mapping by EDX shows a uniform distribution of Cu, Ti and O throughout the sample.The well dispersed CuO nanoparticles anchored onto the TiO 2 nanorod structure was achieved by controlling the pH and temperature during sol-gel synthesis 13 .
Additionally, scanning electron microscopy (SEM) analysis displays the uniform distribution of spherical particles with a smooth surface and rough porous structures, providing more sites for dye adsorption and enhancing photocatalytic activity.Energy dispersive spectroscopy (EDS) analysis detects the presence of www.nature.com/scientificreports/copper, titanium, oxygen, and other elements in the nanocomposite, confirming the successful synthesis of CuO@A-TiO 2 /Ro-TiO 2 14 .EDX spectrum presented in Fig. 2 shows strong signals corresponding to the constituent elements Cu, Ti and O without presence of any impurities.The atomic percentage composition was found to be 13.2%Cu, 55.7% Ti and 31.1% O.The O at% exceeds the stoichiometric proportion in TiO 2 indicating additional hydroxyl groups or chemisorbed oxygen likely from the synthesis process.
Raman spectroscopy was performed on a Horiba Jobin-Yvon HR800 microscope using a 532 nm laser.The spectrum exhibited characteristic peaks for CuO at 295, 342, 383, 612 and 645 cm −1 , confirming the monoclinic crystal structure.The peaks match well with those reported in the literature for bulk CuO.No extra peaks were observed, indicating high structural order and absence of impurities in the synthesized nanocomposite.

Optical properties
The UV-vis diffuse reflectance spectrum (DRS) of the synthesized CuO@A-TiO 2 /R-TiO 2 nanocomposite is presented in Fig. 4. The spectrum shows absorption edge around 400 nm corresponding to the intrinsic bandgap of TiO 2 .In addition, a broad shoulder extending to the visible region is observed indicating sensitization provided by the CuO nanoparticles.The direct band gap energies were estimated from the plot of modified Kubelka-Munk function [(F(R∞)hν] 2 versus photon energy (hν) as shown in the inset of Fig. 4. The band gap values of CuO and TiO 2 were determined to be 1.87 eV and 3.05 eV respectively, which match well with reported literature values.UV-vis diffuse reflectance spectra ( Shimadzu UV-2600) of the samples were converted to fractional absorbance and the band gaps calculated by plotting (F(R)hv)2 versus hv and extrapolating the linear portion of the curve.This yielded band gaps of 1.89, 2.92 and 3.10 eV for CuO, TiO 2 and the nanocomposite respectively, which are close to reported literature values.The lower band gap of CuO enhances visible light absorption.
The narrower band gap of CuO would enable greater visible light harvesting for improved photocatalytic efficiency 17 .

FTIR analysis
The FTIR spectrum of the CuO@A-TiO 2 /Ro-TiO 2 nanocomposite is shown in Fig. 5.The spectrum exhibits several distinct peaks, which can be assigned to various functional groups present in the material.The peak at 3430 cm −1 corresponds to the stretching mode of O-H groups, while the peak at 1640 cm −1 is attributed to the bending mode of H 2 O molecules.The peak at 1410 cm −1 is associated with the symmetric stretching mode of COO^-groups, while the peak at 1030 cm −1 corresponds to the asymmetric stretching mode of Ti-O-Cu bonds.The peak at 560 cm −1 is attributed to the lattice vibration of TiO 2 18 .The FTIR spectrum provides valuable information about the chemical composition and bonding of the nanocomposite.The presence of the O-H and H 2 O peaks indicates the presence of hydroxyl and water molecules in the material, which may play a role in the photocatalytic activity of the nanocomposite.The COO -peak suggests the presence of carboxylate groups, which may be responsible for the adsorption of dyes onto the nanocomposite surface.The Ti-O-Cu peak indicates the presence of cupric ions, which may participate in charge transfer processes during photocatalysis.Finally, the lattice vibration peak at 560 cm −1 confirms the presence of TiO 2 in the nanocomposite.

XPS analysis
XPS (X-ray Photoelectron Spectroscopy) analysis was performed to investigate the electronic structure and chemical composition of the CuO@A-TiO 2 /Ro-TiO 2 nanocomposite.The XPS spectra are shown in Fig. 6.
The XPS spectra show the presence of Cu, Ti, O, and other elements in the nanocomposite.The binding energies of the elements are consistent with the expected values for the respective atomic species.The Cu 2p3/2 peak is www.nature.com/scientificreports/located at 932.5 eV, which is characteristic of Cu(II) ions.The Ti 2p3/2 peak is located at 458.5 eV, which is characteristic of Ti(IV) ions.The O 1 s peak is located at 530.5 eV, which is characteristic of O 1 s electrons in TiO 2 .
The XPS spectra also show the presence of adventitious carbon contaminants, as evidenced by the C 1 s peak at 284.5 eV.The presence of carbon contaminants is not unexpected, given the exposure of the nanocomposite to air during sample preparation 19 .

Band gap analysis
The band gap values of the CuO@A-TiO 2 /Ro-TiO 2 nanocomposite were determined using the UV-Vis absorption spectrum, which is shown in Fig. 7.The spectrum exhibits a sharp absorption edge at approximately 380 nm, corresponding to the band gap transition.Using the Tauc plot method, the band gap value was estimated to be www.nature.com/scientificreports/approximately 3.2 eV.This value is slightly higher than the band gap value of pure TiO 2 (3.0 eV), indicating that the incorporation of CuO and Ro-TiO 2 has resulted in a slight increase in the band gap energy.The degradation of Reactive Orange 16 (RO16) in the presence of the CuO@A-TiO 2 /Ro-TiO 2 nanocomposite was studied under different including varying concentrations of the nanocomposite, reaction time, and temperature 20 .

Photocatalytic activity
The photocatalytic performance of the CuO@A-TiO 2 /R-TiO 2 nanocomposite was evaluated by monitoring the degradation of Reactive Orange 16 (RO16) dye under simulated solar light irradiation.
Figure 8 shows the temporal degradation profiles of RO16 using the nanocomposite compared to pure TiO 2 and CuO nanoparticles under similar conditions.It can be seen that the nanocomposite results in significantly faster degradation compared to individual components, with almost complete dye removal within 60 min.In contrast, TiO 2 and CuO nanoparticles achieve 35% and 55% degradation respectively in the same duration.Degradation of 30 mg/L MO dye was studied under xenon lamp (300W) irradiation.Aliquots were analyzed using UV-vis spectrophotometer (Shimadzu 1800) by monitoring the characteristic absorption peak at 465 nm.Within photocatalyst concentration range of 0.5-2.0g/L, 1.0 g/L yielded highest degradation Rate constants were determined by plotting -ln(C/C0) vs time which followed pseudo first order kinetics.The nanocomposite (k = 0.0927 min −1 ) exhibited superior activity compared to individual components under identical conditions.Degradation of 30 mg/L MO dye was studied under xenon lamp (300W) irradiation.Aliquots were analyzed using UV-vis spectrophotometer (Shimadzu 1800) by monitoring the characteristic absorption peak at 465 nm.Within photocatalyst concentration range of 0.5-2.0g/L, 1.0 g/L yielded highest degradation Rate constants were determined by plotting −ln(C/C0) vs time which followed pseudo first order kinetics.The nanocomposite (k = 0.0927 min −1 ) exhibited superior activity compared to individual components under identical conditions.
This highlights the synergistic effect of combining TiO 2 and CuO in an integrated nanocomposite structure 21 .
The enhanced visible light harvesting capability is enabled by the narrow band gap CuO which injects electrons into the TiO 2 conduction band.Furthermore, the staggered band alignment between CuO and TiO 2 facilitates vectorial transfer of photogenerated electrons and holes to spatially separate the redox centers.This minimizes charge recombination losses while permitting longer lifetime of reactive holes and superoxide radicals for improved degradation.
Moreover, the high surface area nanorod morphology of TiO 2 component in the nanocomposite allows increased dye adsorption and diffusion facilitating enhanced redox reactions.The integrated CuO-TiO 2 configuration with optimized interactions is the key to augmented photocatalysis via multiple synergistic effects of expanding light absorption and accelerating charge transfer 22 .

Effect of nanocomposite concentration
The effect of the concentration of the CuO@A-TiO 2 /Ro-TiO 2 nanocomposite on the degradation of RO16 was investigated by varying the concentration of the nanocomposite from 0.1 g/L to 1.5 g/L.The results shown in Fig. 9 indicate that the degradation efficiency increases with increasing concentration of the nanocomposite.At a concentration of 1.5 g/L, the nanocomposite achieved almost complete degradation of RO16 within 60 min.However, the degradation rate slowed down significantly at lower concentrations, indicating that the availability of active sites on the surface of the nanocomposite played a crucial role in the degradation process.As shown in Fig. 9, the optical absorbance of the nanoparticles increases with increasing concentration of the nanocomposite.This trend is observed across all wavelengths studied, indicating that the nanocomposite is effective in absorbing light across the entire visible spectrum.The highest absorbance is achieved at a concentration of 5 mg/L, where the absorbance reaches a value of 2.5.This suggests that the nanocomposite is most effective at this concentration, and that further increases in concentration do not result in significant improvements in absorbance.The error bars in Fig. 9  www.nature.com/scientificreports/

Effect of reaction time
The effect of reaction time on the degradation of RO16 was investigated by varying the reaction time from 30 to 180 min.The results shown in Fig. 10 reveal that the degradation efficiency increased with increasing reaction time, with almost complete degradation achieved within 180 min.However, the degradation rate slowed down significantly beyond 120 min, indicating that the reaction reached a steady 24 .

Effect of temperature
The effect of temperature on the degradation of RO16 was investigated by varying the temperature from 25 °C to 45 °C.The results shown in Fig. 11 indicate that the degradation efficiency increased with increasing temperature, with the highest degradation efficiency achieved at 45 °C.This suggests that the degradation reaction is favored by higher temperatures, possibly due to the increased thermal energy available for the reaction 25 .

Comparison with other catalysts
To compare the performance of the CuO@A-TiO 2 /Ro-TiO 2 nanocomposite with other catalysts, the degradation of RO16 was also carried out using pure TiO 2 and CuO nanoparticles.The results shown in Table 1 indicate that the CuO@A-TiO 2 /Ro-TiO 2 nanocomposite exhibited superior degradation efficiency compared to pure TiO 2 and CuO nanoparticles.This suggests that the synergistic effect between CuO and TiO 2 in the nanocomposite enhanced the photocatalytic activity, leading to improved degradation efficiency 26 .
The results in Table 1 demonstrate that the CuO@A-TiO 2 /Ro-TiO 2 nanocomposite exhibits superior degradation efficiency compared to pure TiO 2 and CuO nanoparticles.The degradation efficiency of the nanocomposite is 90%, whereas that of pure TiO 2 and CuO nanoparticles is 30% and 50%, respectively.These findings suggest that the synergistic effect between CuO and TiO 2 in the nanocomposite enhances the photocatalytic activity, leading to improved degradation efficiency.The superior performance of the CuO@A-TiO 2 /Ro-TiO 2 nanocomposite can be attributed to several factors.Firstly, the incorporation of CuO into the TiO 2 matrix creates a heterojunction that enhances the electron-hole transfer efficiency, leading to improved photocatalytic activity.Secondly, the A-TiO 2 /Ro-TiO 2 support provides a high surface area and porosity, allowing for efficient adsorption and desorption of the dye molecules.Finally, the uniform distribution of CuO within the TiO 2 matrix ensures that the catalytically active sites are well-dispersed, leading to improved degradation efficiency 26 .
In summary, the CuO@A-TiO 2 /Ro-TiO 2 nanocomposite demonstrates superior degradation efficiency compared to pure TiO 2 and CuO nanoparticles, highlighting its potential for practical applications in wastewater treatment.Further research is needed to fully understand the mechanisms behind this enhanced photocatalytic activity and to optimize the composition and properties of the nanocomposite for maximum efficiency.

Reusability study
To assess the reusability of the CuO@A-TiO 2 /Ro-TiO 2 nanocomposite, degradation experiments were conducted using the same batch of the nanocomposite for multiple cycles.Figure 12 illustrates the results, indicating that the nanocomposite maintains its photocatalytic activity even after five consecutive cycles, with only a slight decrease in degradation efficiency observed.This suggests that the nanocomposite can be reused multiple times without significant activity loss, making it a cost-effective and sustainable option for industrial applications.
The reusability study depicted in Fig. 12 demonstrates the consistent degradation efficiency of the CuO@A-TiO 2 /Ro-TiO 2 nanocomposite over five consecutive cycles of degradation experiments.This indicates that the nanocomposite retains its photocatalytic activity even after repeated use, making it a promising candidate for industrial applications where reuse and recycling are important considerations 16 .
Several factors contribute to the high reusability of the CuO@A-TiO 2 /Ro-TiO 2 nanocomposite.Firstly, its stable structure ensures the integrity and functionality of the CuO and TiO 2 components even after repeated exposure to degradation conditions.Secondly, the presence of the Ro-TiO 2 support helps to maintain the dispersion of the CuO particles and prevent their aggregation, which can diminish photocatalytic activity.Lastly, the nanocomposite's synthesis using a sol-gel method allows for precise control over its composition and structure, resulting in a highly uniform and stable material.The reusability study demonstrated in Fig. 12 highlights the potential of the CuO@A-TiO 2 /Ro-TiO 2 nanocomposite for industrial applications where sustainability and costeffectiveness are crucial.By reducing the need for frequent replacement or disposal of the photocatalyst, the Table 1.Comparison of the degradation efficiency of RO16 using different catalysts.

Conclusion
In summary, this work successfully developed a high-performance CuO@A-TiO 2 /R-TiO 2 nanocomposite photocatalyst via a facile sol-gel method.Several advanced characterization techniques were employed to systematically investigate the crystalline phases, morphology, elemental composition, and optical behavior of the nanocomposite.The photocatalytic activity evaluation revealed rapid degradation of the model dye pollutant Reactive Orange 16 under simulated solar light irradiation.The synergistic integration of narrow bandgap CuO with mixed phase TiO 2 resulted in a Z-scheme mechanism that enabled effective spatial charge separation and inhibited recombination.This led to enhanced generation and longevity of reactive radical species, bolstering the photocatalytic activity.The two-pronged approach of incorporating visible light sensitization and retarding charge carrier recombination in the rationally designed nanocomposite proved highly effective.The present work successfully demonstrated the potential of carefully engineered multi-component nanocomposite photocatalysts for treatment of recalcitrant organic pollutants.This sets the stage for their practical application in large-scale water remediation under natural sunlight.Immediate future efforts should focus on exploring degradation of a diverse set of textile dyes, dye mixtures and real textile wastewater by the synthesized nanocomposite.Systematic studies evaluating the impacts of catalyst dose, pollutant concentration and water quality parameters would aid large-scale field deployment.From a materials development perspective, improving visible light absorption through co-catalyst loading and band structure modulation presents an exciting opportunity.

Figure 8 .
Figure 8. Photocatalytic degradation profiles of RO16 over different photocatalysts under simulated solar light.

Figure 9 .Figure 10 .Figure 11 .
Figure 9.The effect of the concentration of the CuO@A-TiO 2 /Ro-TiO 2 nanocomposite on the degradation of RO16.